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There are theories galore about why some dog breeds appear to be smarter than others, but new research suggests that size alone might make a difference.

All larger dogs appear to be better at following pointing cues from humans than smaller dogs, which makes them appear smarter.

It's possible that bigger dogs appear smarter not just because they are bred for taking orders, but because their wider set eyes give them better depth perception. As a result, they can more easily discern the direction a person is pointing.

This latter hypothesis was tested by researchers in New Zealand, who think there might be something to it.

"We do know that dog breeds are different," said William Helton of the University of Canterbury in Christchurch, New Zealand.

Human breeding has created dogs with huge physical differences, like shorter snouts for more powerful bites. Even the internal structure of dogs eyes can vary among some breeds, he said.

But can something as simple as the distance between the eyes be a factor too?

To see if all larger dogs in general were better at discerning human pointing cues, Helton and his colleagues put 104 dogs to the test -- 61 large dogs (greater than 50 lbs) and 43 small dogs (less than 50 lbs).

The dogs were first briefly trained to retrieve food from a bowl. Then, while a dog was being held by its owner, two bowls with food were placed in front of the dog at the same time. The experimenter then stood six to eight feet away from the dog with folded arms.

After making eye contact with the dog, the researcher pointed with an outstretched finger for less than a second toward a bowl, then refolded his arms. The dog was then released and allowed to go to a bowl. The test was repeated 20 times for each dog.

What Helton found was that larger dogs were clearly better at making the correct choice than the little dogs. What's more, even mid-sized dogs were pretty good at it. This suggests that if the cause of their success is the distance between their eyes, the advantage is limited beyond a certain point, at least in the context of this experiment. The smaller dogs were definitely less able to follow the pointing cue.

The results are published in the September issue of the journal Behavioural Processes.

"Larger dogs should, all other things being equal, have greater inter-ocular distances and this may improve their visual abilities for some tasks," Helton said.

But that's the rub: the "all things being equal." They are not, say other dog researchers.

"There are proven breed differences in dog behavior that reflect hundreds of generations of intentional breeding for roles such as going to where the sheep herder points and that are paired with the body size needed to do the herding work," said dog behaviorist Benjamin Hart of the School of Veterinary Medicine at the University of California, Davis.

"One expects larger breeds, selected for working roles, to be more likely to go to where a handler points," he added.

On the flip side, small breeds like terriers are bred to react quickly and scramble after fast-moving rodents, said Hart.

"The (smaller) reactive breeds are going to be less likely to patiently sit still while a person pointing finally points," Hart said.